Oxidative Stability of Petroleum Oil Fractions

Introduction

The determination of the oxidative induction temperature is a rapid method for assessing the stability of petroleum oil fractions. The same method can be used to measure the effectiveness of stabilizers. Besides this, aging processes can also be investigated. Standardized isothermal methods are often used instead of dynamic methods (e.g. ASTM D 5483 and ASTM E 1858). Analysis under high pressures of oxygen prevents the vaporization of volatile components and increases the rate of oxidation, thereby shortening the measurement time [1, 2].

Samples

Diesel oils of the following petroleum fractions: Light (LGO), Light Cycle (LCGO), Light Vacuum (LVGO), Vacuum1 (VGO1), Vacuum2 (VGO2), Vacuum3 (VGO3) and Kerosine.

Information Expected

Comparison of the products with respect to their stability in oxygen.

Measurement Parameters

Measuring cell: DSC27HP / TC15

Crucible: Aluminum 40 µl, with pierced lid (1 mm hole)

DSC measurement: Put the measuring cell under an atmosphere of oxygen and heat from 40 °C to 150 °C at 20 K/min (to save time), then continue up to 350 °C at 5 K/min. The measurement is automatically terminated when the value of the exothermic DSC signal reaches 10 mW (the combustion peak is not of interest).

Atmosphere: Oxygen at 3 MPa, no purge gas.

Interpretation

The high pressure of oxygen prevents any premature vaporization so that, on heating, a deflection in the DSC curve is caused only by a change in the specific heat of the sample and no other thermal effects are observed before the beginning of oxidation. The full curve for light diesel oil (LGO) is shown in the upper diagram; the change in the heating rate at 150 °C is marked with a vertical line. The lower diagram shows the measurements of all the samples from 150 °C onwards. The curves are arranged so that they can be seen clearly. The tangents used for the evaluation are drawn for the samples of lowest and highest stability. The abbreviations refer to the petroleum fractions men tioned above.

Evaluation

The oxidative stability is compared by determining the temperature at which oxidation begins (onset). This value is considered to be characteristic of the sample. It is measured as the point of intersection of the horizontal tangent of the baseline before oxidation and the tangent at the steepest part of the DSC curve: 

Conclusions

The stability of the various fractions is very different. The DSC curves also show differences at the beginning of oxidation. Similar fractions, however, show almost identical DSC curves with practically the same onset temperatures (e.g. VGO1 and VGO2). This makes it very easy to classify the fractions according to their stability. Long-term stability at low temperatures can, however, only be predicted with kinetic experiments.

_{Oxidative Stability of Petroleum Oil Fractions | Thermal Analysis Application No. UC 103 | Application published in METTLER TOLEDO Thermal Analysis UserCom 10}